1. Technical Field
The present invention relates to a thermoelectric conversion element and a pyroelectric sensor.
2. Related Art
The pyroelectric sensor is roughly divided into a quantum-type sensor and a thermal-type sensor based on detection principles thereof. The thermal-type sensor has attracted attention from the aspect that a cooling step for the noise countermeasure is not necessary. For example, the pyroelectric sensor (a pyroelectric infrared sensor) which is one of the thermal-type sensors has been widely used in a motion sensor or the like, and can obtain information of the existence or temperature of a human body in a contactless manner by detecting infrared radiation in the vicinity of a wavelength of 10 μm which is radiated from the human body. Such a pyroelectric sensor is provided with a thermoelectric conversion element.
The thermoelectric conversion element has properties (pyroelectric effect) which cause temperature change by absorbing infrared thermal energy such that an electrical charge is generated in response to the temperature change. As a material (a pyroelectric material) which forms the thermoelectric conversion element, a material having spontaneous polarization is frequently used. For example, lead zirconate titanate (PZT) can be exemplified as a representative material. It is well-known that it is possible to obtain a relatively large pyroelectric coefficient by using the PZT. A pyroelectric effect is a phenomenon which occurs due to the temperature change of the spontaneous polarization. For this reason, in a case where the material having the spontaneous polarization is used as the pyroelectric material, it is necessary to induce the spontaneous polarization by applying a voltage. In this case, a domain is generated in the material so as to relieve internal stress due to the spontaneous polarization.
For example, a pyroelectric sensor in which PZT formed by using a spin coating method is used as a pyroelectric material has been suggested (refer to JP-T-2003-530538). The PZT basically has a rhombohedral structure, and domains of 71° and 109°.
On the other hand, in a case of using a pyroelectric material having a tetragonal structure, the pyroelectric material has a domain of 90°.
However, in recent years, higher performance of the pyroelectric sensor has been required, and thus more improved sensitivity has been desired, which is a problem to be solved. The problem also exists in a thermoelectric conversion element which is built into a device other than the pyroelectric sensor.